As said before, MS isn't doing generations any more. They want you to be able to play the games you are playing now, 10 years from now if you want. They'll just be upgrades every 4 years or so, but you don't have to upgrade to play the newest game.
Why not refresh now? It's a big jump from the X1, and it seems to be helping the momentum the X1S started. The Xbox brand in general needed a shot in the arm. I think waiting 2 more years would have been a big problem.
Sony is sticking with generations, and I'm really curious to see what they'll do about BC. As far as I understand, their OS makes it more difficult to do..? Anyone please correct me if I'm wrong.
Also, on an unrelated note, I hadn't seen this before: